How to calculate concrete volume
Concrete is measured by volume: Volume = Length × Width × Thickness. Keep the units consistent — for a metric slab, convert thickness from cm to metres (÷100) so the answer is in cubic metres. For imperial, convert thickness from inches to feet (÷12) for cubic feet, then divide by 27 for cubic yards.
How many bags of concrete?
Each pre-mix bag yields a set volume once mixed — roughly 0.017 m³ (0.6 ft³) for an 80 lb bag, less for smaller bags. Divide your total volume (plus waste) by the bag's yield and round up. For anything above about 1 m³ (≈35 ft³), ready-mix delivery is usually cheaper and easier than mixing dozens of bags.
Tips for ordering
- Always add a waste allowance (5–10%) for spillage, over-dig and uneven subgrade.
- Standard slab thicknesses: ~10 cm (4″) for paths and patios, 15 cm (6″)+ for driveways.
- Yields vary by brand — check your bag, and treat bag counts as a close estimate.
Bags or ready-mix?
Bags suit small jobs (a few footings, a small pad). For larger pours, ready-mix delivered by the cubic metre/yard saves time and often money.
What thickness do I need?
Paths/patios ~10 cm (4″); driveways and load areas 15 cm (6″) or more. Follow local codes for structural work.
Does it work for footings and columns?
Yes — treat length × width × depth as the box volume. For round columns, multiply π × radius² × height separately.